1. Field of the Invention
The present invention relates to an image sensing apparatus and an optical navigating apparatus utilizing the image sensing apparatus, and particularly relates to an image sensing apparatus that can avoid overexposure and an optical navigating apparatus with the image sensing apparatus.
2. Description of the Prior Art
A conventional optical navigating apparatus such as an optical mouse or an optical touch control apparatus always generates light to illuminate an object (ex. a finger, a touch control pen, or a surface that an optical navigating apparatus is put on), and then a plurality of frames comprising the object image is captured by an image sensing apparatus to compute location for the object. In order to avoid wrong determining due to the disturbance of environment light, many techniques are developed. One of them is capturing a frame in the period that the light source is turned off, capturing another frame in the period that the light source is turned on, and subtracting these frames to acquire a frame only with the light from the light source, to increase the accuracy for determining object location. The image sensing apparatus comprises a plurality of sensing pixels to generate sensing signals according to sensed light. Above-mentioned frames are generated via processing these sensing signals. FIG. 1 is a circuit diagram illustrating part of the pixel structure for prior art. As shown in FIG. 1, the sensing pixel 100 comprises a light sensing device PD (ex. a photo diode) and a charge storing device (ex. a capacitor). The light sensing device PD generates charges according to received light and stores the charges into the charge storing device C.
FIG. 2 is a schematic diagram illustrating how the charge storing device stores photoelectrons if the light source is turned on and if the light source is turned off, for prior art. As shown in FIG. 2, if the light source LS in FIG. 1 is turned off, the light sensing device PD generates an amount CRe of photoelectrons according to the environment light (the white circles), these photoelectrons are transmitted to the charge storing device and stored. If the light source LS in FIG. 2 is turned on, the light sensing device PD generates an amount CRe+CRL of photoelectrons (black circles mean the photoelectrons generated by the light source light) according to environment light and light source LS. Theses photoelectrons are transmitted to the charge storing device C and stored as well. The photoelectrons generated in the period that the light source is turned on and in the period that the light source is turned off are respectively read out. The image sensing signals are generated accordingly, and then the frames are accordingly generated. By this way, the frame with only the light source light is acquired.
However, if the environment light is too bright, the accuracy for above-mentioned steps may be affected. FIG. 3 is a schematic diagram illustrating the case that the environment light is too bright such that the amount for photoelectrons is larger than the maximum charge storing amount for the charge storing device (i.e. over exposure). As shown in FIG. 3, if the environment light is too bright, the amount for photoelectrons generated by the light sensing PD according to environment light (the white circles) may exceed the maximum charge storing amount for the charge storing device. Or, in another case, the amount for photoelectrons generated by the light sensing PD according to environment light is less than the maximum charge storing amount for the charge storing device, but the sum for the photoelectrons generated by the environment light and the photoelectrons generated the light source light exceeds the maximum charge storing amount for the charge storing device. Either of these situations may cause the inaccuracy for acquiring the frame with only light source light.